1. Technical Field
The present invention relates to a virtual image display apparatus such as a head-mounted display worn on the head and used.
2. Related Art
In recent years, as a virtual image display apparatus that enables formation and observation of a virtual image such as a head-mounted display, various virtual image display apparatuses of a type for guiding video light from a display element to the pupils of an observer with a light guide plate are proposed. As the light guide plate for such virtual image display apparatuses, there is known a light guide plate that guides video light making use of total reflection and reflects the video light on plural partial reflection surfaces arranged in parallel to one another at a predetermined angle with respect to a principal plane of the light guide plate and extracts the video light to the outside from the light guide plate to thereby cause the video light to reach the retinas of the observer (see JP-T-2003-536102 (the term “JP-T” as used herein means a published Japanese translation of a PCT application) and JP-A-2004-157520).
In the virtual image display apparatus explained above, for example, light beams from upper and lower ends of a longitudinal cross-section of the display element need to be made incident on the pupils of the observer at a large tilt angle corresponding to an angle of view. The light beams are emitted from the display element at a relatively large tilt angle. A light beam from the center of the display element is made incident on the pupils of the observer without being tilted from the front. Therefore, the light beam is emitted in the front direction from the display element. When the display element is obtained by combining, for example, an illuminating device and a liquid crystal panel, in general, a distribution of light of the illuminating device is substantially uniform in a screen having an intensity peak in a direction perpendicular to the liquid crystal panel. Therefore, even if luminance is high in the center of an image where the tilt of illumination light is small, luminance falls at upper and lower ends of the image where the tilt of the illumination light increases. This causes luminance spots of the image. In this case, the illumination light is not effectively utilized at upper and lower ends of the screen. It can be said that efficiency of use of the illumination light falls.
On the other hand, concerning a light beam from a lateral cross-section of the display element, an angle direction in which the light beam is captured by the light guide plate and used for display substantially tilts from the front direction of the display element according to a lateral position of the light beam. Therefore, a phenomenon could occur in which, even if luminance is high in a lateral position where the tilt of the illumination light is small, luminance falls in another lateral position where the tilt of the illumination light increases. In this case, the illumination light is not efficiently used and belt-like luminance spots extending in the longitudinal direction occur.